The Long Term Evolution-Advanced (LTE-A) network is designed to improve the spectral efficiency by reducing the cell size through utilizing a heterogeneous deployment of a diverse set of base stations (BSs). FIG. 1 illustrates an exemplary heterogeneous cellular network, where the macro 100 and pico 101 BSs coexist to serve user equipments (UEs) 102 in the area. The macro BSs are deployed in a regular and planned manner with high transmit power (the typical value is 46 dBm to cover a macro cell 103) and the overlaid pico BSs are deployed in areas with poor coverage (e.g., providing a pico cell 104 to cover the edge of a macro cell) with relatively low transmit power (the typical value is 30 dBm). Such an overlaid BS deployment can improve the coverage and provide capacity gain by increasing spatial reuse of the spectrum.
When a user equipment (UE) is turned on, it searches for a suitable cell (which could either be a macro or pico cell in a heterogeneous cellular network) with which to associate. To determine which cell to select, a UE measures the reference signals (RSs) from the BSs in its surrounding area. Based on the reference signal received power (RSRP), the UE associates itself to the BS with the maximum RSRP.
FIG. 2 illustrates an example of a UE measuring reference signals from a macro BS and a pico BS. A UE 200 measures the reference signals RS1 and RS2 from the macro BS 201 and pico BS 202, respectively. When the RSRP of RS1 is greater than that of RS2, the UE 200 associates itself to the macro BS as shown at 203. The UE 200 undergoes a cell selection process periodically. In a subsequent period, if the RSRP of RS1 is less than that of RS2, the UE 200 will choose the pico cell 202 as its serving cell instead. Note that a UE 200 reports measurement information periodically to its associated cell, which includes the RSRP values for the other BSs in its neighborhood. Such information will be used for future cell selection.
In a heterogeneous network, the pico UEs that are served by the pico BSs suffer severe interference from the macro BSs due to their high transmit powers. In order to reduce the interference to the pico UEs, the macro BSs can mute certain subframes, which are called almost blank subframes (ABSs). In an Almost Blank Subframe (ABS), most resource elements (REs) are blank and only a small amount of REs carry some system information (e.g., cell-specific RSs and synchronization signals). The pico UEs can achieve a higher data rate when the macro BSs transmit ABSs due to the reduced interference level from the muted subframes.
FIG. 3 illustrates an exemplary LTE-A frame. The LTE-A frame 300 can be divided into subframes 301. An LTE-A frame typically contains 10 subframes that are indexed from 0 to 9. For the ABS configuration with the macro BSs, subframes 1, 2, 7, and 8 are configured as ABS in this example. Note that the ABS pattern configured by one BS can be indicated to neighboring BSs via an X2 interface in the LTE-A network. Based on the ABS pattern configured by the macro BSs, the pico BSs transmit data packets to their cell-edge UEs in subframes 1, 2, 7, and 8 and serve their cell-center UEs in the rest of the subframes, such that the overall throughput of the pico UEs (especially the cell-edge UE throughput) is improved.